1. The Field of the Invention
The present invention generally relates to support systems for use in conjunction with components designed for rotary motion. More particularly, the present invention relates to a rotary component support system for a rotating anode of an x-ray tube.
2. Related Technology
X-ray generating devices are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. For example, such equipment is commonly employed in areas such as medical diagnostic examination and therapeutic radiology, semiconductor fabrication, and materials analysis.
Regardless of the applications in which they are employed, most x-ray generating devices operate in a similar fashion. X-rays are produced in such devices when electrons are accelerated to high speeds and then impinged upon a material of a particular composition. This process typically takes place within an evacuated housing of an x-ray tube located in the x-ray generating device. The x-ray tube includes an electron source, or cathode, and an anode oriented to receive electrons emitted by the cathode. The anode, which typically comprises a graphite substrate and a heavy metallic target surface, can be stationary within the tube, or can be in the form of a rotating disk supported by a bearing assembly and a support shaft.
In operation, an electric current is supplied to a filament portion of the cathode, causing it to emit a stream of electrons by thermionic emission. A high electric potential placed between the cathode and anode causes the electron stream to accelerate toward a target surface located on the anode. Upon striking the target surface, some of the resulting kinetic energy is released as electromagnetic radiation of very high frequency, i.e., x-rays. The specific frequency of the x-rays produced depends in large part on the type of material used to form the anode target surface. Target surface materials having high atomic numbers (xe2x80x9cZ numbersxe2x80x9d), such as tungsten, are typically employed. The x-rays ultimately exit the x-ray device through a window formed in the x-ray tube housing so as to interact in or on material samples or patients. As is well known, the x-rays can be used for sample analysis procedures, therapeutic treatment, or in medical diagnostic procedures.
In general, only a small portion of the kinetic energy contained in the electron stream is converted into x-rays. A majority of the energy is dissipated as heat in the anode target region and the rest of the anode. This heat can reach extremely high temperatures that can damage the anode structure over time, and can reduce the operating life of the x-ray tube and/or the performance and operating efficiency of the tube. To help alleviate this problem, the x-ray target, or focal track, is typically positioned on an annular portion of a rotatable anode disk. Typically, the anode disk (also referred to as the rotary target or the rotary anode) is mounted on a supporting shaft which, in turn, is supported by bearings contained a bearing housing. The shaft and disk are then appropriately connected to and rotated by a motor.
When the anode is rotated, the focal track is rotated into and out of the path of the electron beam. In this way, the electron beam is in contact with specific points along the focal track for only short periods of time, thereby allowing the remaining portion of the track to cool during the time that it takes the portion to rotate back into the path of the electron beam.
While the basic operational principles of x-ray devices have remained substantially unchanged, new uses and applications for x-rays have increased the performance demands placed on x-ray tubes. One response to such demands has resulted in the development of anodes that have increasingly larger sizes and/or that are suited for relatively higher operational speeds. While such anodes generally facilitate a desirable increase in the overall performance of the x-ray tube, the increased size and/or speed of those anodes often can cause other undesirable problems with respect to other portions of the x-ray tube. An area of particular concern in this regard is the mounting system used to rotatably support the anode.
In general, the quality of the images produced by a particular device is at least partially a function of the stability of the anode. In particular, image quality can depend on the changes in the relative position of the focal spot, or the point at which the electrons strike the target. Any migration of the focal spot due to vibration or other instabilities can reduce the image quality of the x-ray tube. Thus, the anode mounting system must be constructed and implemented in such a way that balanced mounting of the anode can be achieved and maintained, and undesirable movement of the anode during x-ray tube operation thereby minimized or prevented. As suggested above however, such results are not achieved in all cases due to relative increases in the size and weights of anodes now in use, the operational speeds at which such anodes are employed, and/or the extreme thermal stresses imposed on the anode and its mounting system.
In particular, the weights and operational speeds of many anodes can cause various components of the anode mounting system to loosen over time. This effect may be further exacerbated by thermal effects resulting from the high temperature operating environment typical of x-ray devices. Further, anodes typically accelerate at a high rate of revolutions per minute to achieve an operational speed. Such high rates of acceleration introduce various mechanical stresses and strains that often compromise the integrity of the anode mounting system components, and may cause such components to loosen over a period of time.
A related concern with anode mounting systems deals with the extent to which they can be disassembled. In particular, it is often desirable to remove an anode from an x-ray tube after a given amount of operating time, and then recondition and re-install the anode. Removal may also be needed to access other x-ray tube components. However, removal of the anode from the x-ray tube, has often proven difficult. These difficulties are often associated with the configuration and layout of the anode mounting system.
For example, some x-ray tubes employ an anode mounting system that includes a bearing assembly and stationary shaft configured so that so that the anode, rotatably supported by the bearing assembly, rotates about the stationary shaft. In such arrangements, the stationary support shaft is typically mounted to the vacuum enclosure within an aperture or sleeve. In such a configuration, a braze joint usually serves to secure the stationary shaft to the vacuum enclosure and also to seal the vacuum enclosure. This configuration can be especially difficult to disassemble. This is due in large part to the manner by which the stationary shaft is mounted to the vacuum enclosure. To maintain rigid support for the anode, the fit between the sleeve and the support shaft must be extremely tight. Typically, this is achieved with an interference fit, where the sleeve, for example, is heated while the shaft is cooled. Due to the expansion of the sleeve and the contraction of the shaft, the shaft can be inserted into the sleeve. As the two components reach thermal equilibrium, the gap between the shaft and sleeve is eliminated, resulting in a tight fit between the two.
While this is effective in tightly fitting the shaft and sleeve together, it also creates a bond between the sleeve and the support shaft that is very difficult to break. Further, when the bond is broken, damage may occur to the shaft, sleeve, anode, and/or vacuum enclosure. Consequently, disassembly can result in added expense when repairing or replacing tube components and may even render the entire x-ray tube inoperable. While it may be possible in some instances to minimize the damage that may occur during separation of the shaft and sleeve, the processes necessary to achieve such results are typically expensive and time consuming.
In view of the foregoing, and other, problems in the art, a need exists for a rotary component support system that securely and reliably supports the anode in a desired position under a variety of operating conditions. In addition, it would be an advancement to provide a support system that is configured to be readily and non-destructively disassembled, thereby facilitating repair or maintenance of the anode or other components located within the vacuum enclosure.
The present invention has been developed in response to the current state of the art, and in particular, in response to these and other problems and needs that have not been fully or adequately addressed. Briefly summarized, embodiments of the present invention are directed to a rotary component support system that securely and reliably supports a component in a desired position under a variety of operating conditions, and that is configured to be readily and non-destructively disassembled.
Embodiments of the invention are particularly useful in anode grounded x-ray tubes, such as are commonly employed in computerized tomography (xe2x80x9cCTxe2x80x9d) and other medical applications. In general however, embodiments of the invention are suitable for use in any application where it is desired to securely and reliably support a rotatable component in a desired position under a variety of operating conditions, and where it is desired to reduce or minimize the cost and expense associated with the disassembly of the rotary component support system.
Embodiments of the rotary component support system are suitable for use in conjunction with a bearing assembly having a rotatable cylinder to which an anode is mounted. The rotatable cylinder is arranged for rotary motion about a stationary support shaft so that as the cylinder rotates, the anode that is mounted to the cylinder rotates about the support shaft.
The support shaft of the rotary component support system, as well as the anode and bearing assembly, are disposed within the vacuum enclosure of an x-ray tube. In one preferred embodiment, one end of the support shaft is disposed within the bearing assembly. The other end of the support shaft defines a threaded cavity and is tapered on the outside so that it can be received within a mounting piece having a corresponding geometry. The mounting piece is preferably welded within an aperture that is formed through a wall of the x-ray tube housing that defines the vacuum enclosure. Generally, the mounting piece defines a cavity having a taper that corresponds to the geometry of the taper of the support shaft. Also, a threaded hole is formed through a center portion of the mounting piece. A bolt passes through and engages the threaded hole of the mounting piece. The length of the bolt allows it to extend through the hole of the mounting piece so as to likewise engage the threaded cavity defined by the support shaft.
To assemble the rotary component mounting system, the bolt is tightened so that the support shaft is drawn into the mounting piece until the tapered end of the support shaft is seated within the tapered cavity defined by the mounting piece. Sealing of the vacuum enclosure is accomplished by placing a weld at the interface between the bolt and mounting piece.
Because the mounting piece and support shaft have corresponding tapered geometries, the shaft can be readily and quickly aligned with the mounting piece without the need for time consuming adjustments. Alignment of the shaft contributes to the elimination of vibration and other undesirable phenomena often associated with unbalanced rotating components. Further, the bolt cooperates with the tapered geometries of the mounting piece and shaft to facilitate ready separation of the mounting piece and shaft should such a need arise. In particular, the weld on the interface between the bolt and mounting piece can be easily ground off, or otherwise removed, so that disassembly is accomplished by removing the bolt and pulling the mounting piece and support shaft apart from each other.
These and other advantages and features of the present invention will become more fully apparent from the following description and appended claims.